1. Field of the Invention
The present invention generally relates to micro movable devices and to methods of making the same, in particular, methods using wet etching technique.
2. Description of the Related Art
In the technical field of radio communications equipment such as mobile phones, there is an increasing requirement for smaller high-frequency circuit or RF circuit due to increase in the number of incorporated parts for advanced features. In response to such a requirement, a variety of parts needed for building circuitry are a focus of miniaturization using technologies called MEMS (micro-electromechanical systems).
One of these part categories is MEMS switches. MEMS switches are switching devices having a minute structure manufactured by means of MEMS technology, and include at least a pair of contacts for mechanical opening/closing operations to achieve switching, a drive mechanism for achieving the mechanical opening/closing operations of the contact pair and so on. As compared to other switching devices provided by a PIN diode, an MESFET and so on, MEMS switches tend to exhibit higher isolation when the switch is open, and low insertion loss when the switch is closed because MEMS switch contacts are mechanically opened when the switch is in the open state, and mechanical switches are not susceptible to a large parasitic capacitance. MEMS switches are disclosed in the following Patent Document 1 and Patent Document 2 for example.
Patent Document 1: JP-A-H9-17300
Patent Document 2: JP-A-2001-143595
FIG. 17 and FIG. 18 show a micro switching device X2 which is a conventional MEMS switch. FIG. 17 is a partial plan view of the micro switching device X2 whereas FIG. 18 is a sectional view taken in lines XVIII-XVIII in FIG. 17. The micro switching device X2 includes a substrate S2, a fixed portion 51, a movable portion 52, a movable contact 53, a pair of fixed contact electrodes 54, and drive electrodes 55, 56. The fixed portion 51 is bonded to the substrate S2. The movable portion 52 extends from the fixed portion 51 along the substrate S2. The movable contact 53 is provided in the movable portion 52, on a side facing the substrate S2. The drive electrode 55 is provided on the fixed portion 51 and the movable portion 52. The fixed contact electrodes 54 are patterned on the substrate S2 so that an end of each electrode faces the movable contact 53. The drive electrode 56 is provided on the substrate S2 correspondingly to the drive electrode 55, and is grounded. Also on the substrate S2 is a predetermined wiring pattern (not illustrated) which is electrically connected with the fixed contact electrodes 54 or the drive electrode 56.
In the micro switching device X2 having such a structure as the above, when a predetermined electric potential is applied to the drive electrode 55, an electrostatic pull is generated between the drive electrodes 55, 56. As a result, the movable portion 52 is deformed elastically until the movable contact 53 makes contact with the fixed contact electrodes 54, bringing the micro switching device X2 into a closed state. In the closed state, the movable contact 53 electrically bridges the pair of fixed contact electrodes 54, allowing an electric current to pass through the pair of fixed contact electrodes 54.
On the other hand, if the electrostatic pull acting between the drive electrodes 55, 56 is ceased when the micro switching device X2 is in the closed state, the movable portion 52 returns to its natural state, allowing the movable contact 53 to come away from the fixed contact electrodes 54. Thus, as shown in FIG. 18, the micro switching device X2 is brought to an open state. In the open state, the fixed contact electrodes 54 are electrically disconnected from each other, so no electric current can pass through the pair of fixed contact electrodes 54.
FIG. 19 and FIG. 20 show a method of making the micro switching device X2. In the manufacture of the micro switching device X2, first, as shown in FIG. 19(a), fixed contact electrodes 54 and a drive electrode 56 are patterned on a substrate S2. Specifically, a film of predetermined conductive material is formed on the substrate S2, then a predetermined resist pattern is formed on the conductive film by means of photolithography, and an etching process is performed to the conductive film using the resist pattern as a mask. Next, as shown in FIG. 19(b), a sacrifice layer 57 is formed. Specifically, a sputtering method is used for example, to deposit or grow a predetermined material on the substrate S2 while covering the fixed contact electrodes 54 and the drive electrode 56. Next, an etching process is performed using a predetermined mask, to form a recess 57a in the sacrifice layer 57 as shown in FIG. 19(c), correspondingly to the fixed contact electrodes 54. Next, a film of a predetermined material is formed in the recess 57a, whereby a movable contact 53 is formed as shown in FIG. 19(d).
Next, as shown in FIG. 20(a), a structural film 58 is formed by e.g. sputtering method. Next, as shown in FIG. 20(b), a drive electrode 55 is patterned on the structural film 58. Specifically, a film of a predetermined conductive material is formed on the structural film 58, then a photolithographic method is used to form a predetermined resist pattern on the conductive film, and an etching process is performed to the conductive film using the resist pattern as a mask. Next, as shown in FIG. 20(c), the structural film 58 is patterned to form a film piece 59 which contains part of the fixed portion 51 and the movable portion 52. Specifically, a photolithographic method is used to form a predetermined resist pattern on the structural film 58, and then and an etching process is performed to the structural film 58, using the resist pattern as a mask. Next, as shown in FIG. 20(d), a fixed portion 51 and a movable portion 52 are formed. Specifically, a wet etching process is performed to the sacrifice layer 57 to form an undercut below the movable portion 52 while leaving part of the sacrifice layer 57 as part of the fixed portion 51, using the film piece 59 as an etching mask.
One of the characteristics generally required of switching devices is a low insertion loss in the closed state. In order to lower the insertion loss in switching devices, a pair of fixed contact electrodes should have a low electric resistance.
However, according to the micro switching device X2, it is difficult to make thick contact electrodes 54. Realistically, the thickness of the fixed contact electrodes 54 is up to 2 μm at the best because in the manufacturing process of the micro switching device X2, it is necessary to make sure that the sacrifice layer 57 has a certain level of flatness on its upper surface as in the figure (growing surface).
As was described with reference to FIG. 19(b), the sacrifice layer 57 is formed when a predetermined material deposits or grows on the substrate S2 while covering a pair of fixed contact electrodes 54. Therefore, the growing surface of the sacrifice layer 57 will have steps (not illustrated) following the thickness of the fixed contact electrodes 54. The steps will become more bumpy as the fixed contact electrodes 54 is thicker, and as the steps become more bumpy, it becomes more difficult to form the movable contact 53 at an appropriate location, to form the movable portion 52 into an appropriate shape, and so on. Further, if the fixed contact electrodes 54 are thicker than a certain limit, there can be a case in which the sacrifice layer 57 formed on top of the substrate S2 is cracked due to the thickness of the fixed contact electrodes 54. If the sacrifice layer 57 is damaged, it becomes impossible to form a movable contact 53 and/or a movable portion 52 appropriately on the sacrifice layer 57. Therefore, it is necessary in the micro switching device X2 that the fixed contact electrodes 54 are formed thinly enough so that there is no undesirable step on the growing surface of the sacrifice layer 57. Thus, in the micro switching device X2, it is sometimes difficult to render the fixed contact electrodes 54 a sufficiently low resistance, and as a result, it is sometimes impossible to achieve a low insertion loss.
FIG. 21 through FIG. 25 show a micro switching device X3 which is essentially disclosed in a Japanese Patent Application (No. 2005-023388) filed earlier by the applicant of the present invention. The micro switching device X3 relates to an invention aimed at providing a micro switching device suitable for lowering the insertion loss and adequate for manufacture. The earlier application which makes disclosure essentially of the micro switching device X3 was not public before the present application was filed. FIG. 21 is a plan view of the micro switching device X3 whereas FIG. 22 is a partially non-illustrated plan view of the micro switching device X3. FIG. 23 through FIG. 25 are sectional views taken in lines XXIII-XXIII, XXIV-XXIV, and XXV-XXV respectively in FIG. 21.
The micro switching device X3 includes a base substrate S3, a fixed portion 61, a movable portion 62, a movable contact 63, a pair of fixed contact electrodes 64 (not illustrated in FIG. 22), and a piezoelectric drive 65.
As shown in FIG. 23 through FIG. 25, the fixed portion 61 is bonded to the base substrate S3 via a border layer 61′. The fixed portion 61 is made of silicon material such as monocrystal silicon. The border layer 61′ is made of silicon oxide. As shown in FIG. 22 for example, the movable portion 62, has a fixed end 62a fixed to the fixed portion 61, and extends along the base substrate S3 as shown in FIG. 25, and is surrounded by the fixed portion 61, via a slit 66. The movable portion 62 has a body 62A and a head 62B. The movable portion 62 is made of silicon material such as monocrystal silicon.
As clearly shown in FIG. 22, the movable contact 63 is provided on the head 62B of the movable portion 62. As shown in FIG. 23 and FIG. 25, each of the fixed contact electrodes 64 is erected on the fixed portion 61, and has a contact region 64a which faces the movable contact 63. Each of the fixed contact electrodes 64 is connected with a predetermined circuit which is served by the switching device, via a predetermined wiring (not illustrated). The movable contact 63 and the fixed contact electrodes 64 are preferably made of a precious metal selected from a group consisting of Au, Pt, Pd and Ru, or an alloy containing the selected precious metal.
The piezoelectric drive 65 includes electrode films 65a, 65b and a piezoelectric film 65c between the two. Each of the electrode films 65a, 65b has a laminate structure provided by e.g. a Ti underlayer and a Pt main layer. The electrode film 65b is grounded via a predetermined wiring (not illustrated). The piezoelectric film 65c is made of a piezoelectric material which is a material distinguished by a nature (inverse piezoelectric effect) that the material is distorted upon application of an electric field. Examples of the usable piezoelectric material include PZT (a solid solution of PbZrO3 and PbTiO3), ZnO doped with Mn, ZnO and AlN. The electrode films 65a, 65b have a thickness of e.g. 0.55 μm, whereas the piezoelectric film 65c has a thickness of e.g. 1.5 μm.
In the micro switching device X3 which has the structure as described above, when a predetermined positive electric potential is applied to the electrode film 65a, an electric field is generated between the electrode film 65a and the electrode film 65b, and a contractive force is generated in the piezoelectric film 65c in its in-plane directions. Shrinkage of the piezoelectric material in the in-plane directions of the piezoelectric film 65c is greater at a place farther away from the electrode film 65a which is supported directly by the movable portion 62, i.e. there is more shrinkage at a place closer to the electrode film 65b. For this reason, the amount of in-plane shrinkage resulting from the above-described contractive force gradually increases from the side closer to the electrode film 65a toward the side closer to the electrode film 65b, within the piezoelectric film 65c, making the movable portion 62 elastically deform to bring the movable contact 63 into contact with the fixed contact electrodes 64 or the contact region 64a. This brings the micro switching device X3 into a closed state. In the closed state, the movable contact 63 bridges the pair of fixed contact electrodes 64, allowing an electric current to pass through the pair of fixed contact electrodes 64. In such a way, it is possible to make an ON state of e.g. a high-frequency signal.
Now, the micro switching device X3 being in the closed state, when the electric field between the electrode film 65a and the electrode film 65b is ceased by stopping the application of the electric potential to the piezoelectric drive 65, the piezoelectric film 65c and the movable portion 62 return to their natural states, allowing the movable contact 63 to come away from the fixed contact electrodes 64. Thus, the micro switching device X3 is brought to an open state. In the open state, the fixed contact electrodes 64 are electrically disconnected from each other, so no electric current can pass through the pair of fixed contact electrodes 64. In this way, it is possible to make an OFF state of the high-frequency signal.
FIG. 26 through FIG. 29 show a method of manufacturing the micro switching device X3. The figures show changes in part of the section taken in lines XXIII-XXIII in FIG. 21, as well as changes in part of the section taken in lines in FIG. 21. In the manufacture of the micro switching device X3, first, a substrate 70 as shown in FIG. 26(a) is prepared. The substrate 70 is an SOI (silicon on insulator) substrate, having a laminate structure provided by a first layer 71, a second layer 72 and a middle layer 73 between the two. As examples, the first layer 71 has a thickness of 10 μm, the second layer 72 has a thickness of 400 μm and the middle layer 73 has a thickness of 2 μm. The first layer 71 is made of e.g. monocrystal silicon, from which the fixed portion 61 and the movable portion 62 as described above are to be formed. The second layer 72 is made of e.g. monocrystal silicon, from which the substrate S3 is to be formed. The middle layer 73 is made of silicon oxide, from which the border layer 61′ is to be formed.
Next, as shown in FIG. 26(b), a piezoelectric drive 65 is formed on the first layer 71 of the substrate 70. In forming the piezoelectric drive 65, first, a first conductive film is formed on the first layer 71, for formation of an electrode film 65a. Next, a film of a piezoelectric material is formed on the first conductive film, for formation of a piezoelectric film 65c. Next, a second conductive film is formed on the film of piezoelectric material, for formation of an electrode film 65b. Thereafter, each film is patterned in an etching process using a predetermined mask. The first and the second conductive films as the targets of the patterning can be formed by sputtering method for example, by first forming a film of Ti and then forming a film of Pt thereon. The film of piezoelectric material can be formed by sputtering method for example, by forming a film of a predetermined piezoelectric material.
Next, as shown in FIG. 26(c), a movable contact 63 is formed on the first layer 71. This can be done for example, by first forming a film of Cr on the first layer 71 by sputtering, and then forming thereon a film of Au. Next, a photolithographic method is used to form a predetermined resist pattern on this multi-layered conductive film, and then an etching process is performed to the multi-layered conductive film, using the resist pattern as a mask. In this way, it is possible to pattern the movable contact 63 on the first layer 71.
Next, as shown in FIG. 26(d), a protective film 81 is formed for covering the piezoelectric drive 65. The formation of the protective film 81 can be achieved for example, by forming a film of Si using a sputtering method using a predetermined mask. The piezoelectric drive 65, and particularly the piezoelectric film 65c thereof, tends to be eroded by an etchant which is used in the wet etching process employed for removing a sacrifice layer 82 to be described later and part of the middle layer 73. The protective film 81 is formed to protect the piezoelectric drive 65 or the piezoelectric film 65c from the erosion, and is resistant to the etchant.
Next, as shown in, FIG. 27(a), the first layer 71 is etched to form a slit 66. Specifically, a photolithographic method is used to form a predetermined resist pattern on the first layer 71, and then an etching process is performed to the first layer 71 using the resist pattern as a mask. Through this removal step, the first layer 71 yields a fixed portion 61 and a movable portion 62.
Next, as shown in FIG. 27(b), a sacrifice layer 82 is formed on the substrate 70, on the side formed with the first layer 71, to fill the slit 66. The sacrifice layer can be formed of silicon oxide. The sacrifice layer 82 can be formed by means of plasma CVD method or sputtering method for example. In the present step, the sacrifice layer material deposits on part of side walls of the slit 66 as well, and fills the slit 66.
Next, as shown in FIG. 27(c), two recesses 82a are formed in the sacrifice layer 82, at places corresponding to the movable contact 63. Specifically, a photolithographic method is used to form a predetermined resist pattern on the sacrifice layer 82, and then an etching process is performed to the sacrifice layer 82 using the resist pattern as a mask. The etching can be achieved by means of wet etching. Each of the recesses 82a is for formation of a contact region 64a of the fixed contact electrode 64.
Next, as shown in FIG. 28(a), the sacrifice layer 82 is patterned to form open regions 82b. Specifically, a photolithographic method is used to form a predetermined resist pattern on the sacrifice layer 82, and then an etching process is performed to the sacrifice layer 82 using the resist pattern as a mask. The etching can be achieved by means of wet etching. The exposed open regions 82b are regions the fixed portion 61 which the fixed contact electrodes 64 are bonded to.
Next, in the object structure shown in FIG. 28(c), an undercoating film (not illustrated) which serves as an electric path is formed on the surface provided with the sacrifice layer 82 and then, a resist pattern 83 is formed as shown in FIG. 28(b). The undercoating film can be formed by sputtering method for example, by first forming a film of Cr to a thickness of 50 nm and then forming a film of Au thereon to a thickness of 500 nm. The resist pattern 83 has open regions 83a corresponding to the pair of fixed contact electrodes 64.
Next, as shown in FIG. 28(c), the pair of fixed contact electrodes 64 is formed. Specifically, an electroplating method is used to grow a film of e.g. Au on the undercoating film exposed on the open regions 83a. 
Next, as shown in FIG. 29(a), the resist pattern 83 is removed by etching. Thereafter, exposed portions of the undercoating film are removed by etching. These etching processes can be achieved by means of wet etching, each using a predetermined etchant.
Next, as shown in FIG. 29(b), the sacrifice layer 82 and part of the middle layer 73 are removed. Specifically, a wet etching process is performed to the sacrifice layer 82 and the middle layer 73. The etchant can be provided by buffered hydrofluoric acid (BHF). In this etching process, the sacrifice layer 82 is removed first and thereafter, part of the middle layer 73 is removed from places exposed to the slit 66. The etching process ceases when an appropriate gap is formed between the entire movable portion 62 and the second layer 72. FIG. 29(b) shows a gap G, which was formed by the etchant that entered from the slit 66, along the fixed contact electrodes 64 and etched the middle layer 73. In this way, it is possible to etch the middle layer 73 to leave a border layer 61′. Note that the second layer 72 will constitute the base substrate S3.
Next, a wet etching process is performed as necessary, to remove part of the undercoating film (e.g. Cr film) remaining on the bottom surface of the fixed contact electrodes 64, and then the entire device is dried. Thereafter, as shown in FIG. 29(c), the protective film 81 is removed. The removal can be made by e.g. RIE (Reactive Ion Etching) which uses O2 gas as an etching gas.
By following the above-described steps, it is possible to manufacture the micro switching device X3. According to the above-described method, the fixed contact electrodes 64 which have the contact regions 64a faced by the movable contact 63 can be formed thickly on the sacrifice layer 82 by plating method as described with reference to FIG. 28(c). Therefore, it is possible to secure a sufficient thickness for the fixed contact electrodes 64. The micro switching device X3 as the above is suitable for lowering the insertion loss in the closed state. In addition, according to the micro switching device X3, the bottom surface of the contact regions 64a in the fixed contact electrodes 64 (i.e. the surface which can make contact with the movable contact 63) is not a frontier surface where a layer of plated metal grows, and therefore is highly flat. Thus, it is possible to form an air gap between the movable contact 63 and the contact regions 64a at a high dimensional accuracy. The gap which has a high dimensional accuracy is suitable for lowering the insertion loss in the closed state and is suitable for improving isolation characteristics in the open state.
However, according to the above-described method of making the micro switching device X3, there is a case in which the etchant erodes the piezoelectric drive 65 in the step described with reference to FIG. 29(b). Specifically, the wet etching process described with reference to FIG. 29(b) requires a few hours of time. During such a long period of time of the wet etching process, the etchant can gradually make its way along the bonding border surface between the first layer 71 and the protective film 81, and sometimes the etchant reaches the piezoelectric drive 65, in which case the piezoelectric drive 65 (and the piezoelectric film 65c in particular) is eroded by the etchant. The erosion of the piezoelectric drive 65 or the piezoelectric film 65c is undesirable since it prevents the piezoelectric drive 65 from driving the movable portion 62.